PUBLICATIONS Journal of Geophysical Research: Planets RESEARCH ARTICLE Retrieval of Compositional End-Members From Mars 10.1002/2017JE005339 Exploration Rover Opportunity Observations Key Points: in a Soil-Filled Fracture in Marathon Valley, • A log maximum likelihood method was used to determine end-member Endeavour Crater Rim compositions of a fracture in Marathon Valley, Endeavour crater, N. T. Stein1 , R. E. Arvidson2 ,J.A.O’Sullivan3, J. G. Catalano2 , E. A. Guinness2 , Mars 3 4 4 • A Mg-sulfate-rich soil end-member D. V. Politte , R. Gellert , and S. J. VanBommel and hematite-rich pebbles in the 1 2 fracture imply alteration in an Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA, Department of Earth 3 acid-sulfate environment and Planetary Sciences, Washington University in Saint Louis, Saint Louis, MO, USA, Department of Electrical Engineering, • Results add to growing evidence of Washington University in Saint Louis, Saint Louis, MO, USA, 4Department of Physics, University of Guelph, Guelph, ON, alteration along the rim of Endeavour Canada crater by enhanced fluid flow along fracture zones Abstract The Opportunity rover investigated a gentle swale on the rim of Endeavour crater called Marathon Valley where a series of bright planar outcrops are cut into polygons by fractures. A wheel scuff Correspondence to: performed on one of the soil-filled fracture zones revealed the presence of three end-members identified on N. T. Stein, the basis of Pancam multispectral imaging observations covering ~0.4 to 1 μm: red and dark pebbles, and a [email protected] bright soil clod. Multiple overlapping Alpha Particle X-ray Spectrometer (APXS) measurements were collected on three targets within the scuff zone. The field of view of each APXS measurement contained various Citation: proportions of the Pancam-based end-members. Application of a log maximum likelihood method for Stein, N. T., Arvidson, R. E., O’Sullivan, J. A., Catalano, J. G., Guinness, E. A., Politte, retrieving the composition of the end-members using the 10 APXS measurements shows that the dark D. V., … VanBommel, S. J. (2018). pebble end-member is compositionally similar to average Mars soil, with slightly elevated S and Fe. In Retrieval of compositional end-mem- contrast, the red pebble end-member exhibits enrichments in Al and Si and is depleted in Fe and Mg relative bers from Mars Exploration Rover Opportunity observations in a soil-filled to average Mars soil. The soil clod end-member is enriched in Mg, S, and Ni. Thermodynamic modeling of the fracture in Marathon Valley, Endeavour soil clod end-member composition indicates a dominance of sulfate minerals. We hypothesize that acidic crater rim. Journal of Geophysical fluids in fractures leached and oxidized the basaltic host rock, forming the red pebbles, and then evaporated Research: Planets, 123, 278–290. https:// doi.org/10.1002/2017JE005339 to leave behind sulfate-cemented soil. The Opportunity rover investigated a region on the rim of Endeavour Received 3 MAY 2017 Plain Language Summary Accepted 19 DEC 2017 crater on Mars called Marathon Valley where a series of bright outcrops are cut by fractures. A scuff Accepted article online 29 DEC 2017 performed by one of the rover wheels on the fractures revealed the presence of three different compositional Published online 30 JAN 2018 end-members. A novel technique was applied to retrieve the composition of the end-members using measurements by the rover’s Alpha Particle X-ray Spectrometer and Pancam instruments. The presence of a magnesium-sulfate-rich soil end-member and hematite-rich pebbles in the scuffed fracture imply alteration in an acid-sulfate environment. Results add to growing evidence of alteration along the rim of Endeavour crater that was concentrated along fractures, which likely provided a conduit for subsurface fluid flow. The timing of formation of these altered deposits is unclear; they could have formed during transient postimpact hydrothermal activity or perhaps significantly later utilizing groundwater from the southern highlands. 1. Introduction The Opportunity rover arrived at the western rim of Endeavour crater in August 2011. Opportunity’s explora- tion of this highly degraded, 22 km diameter Noachian-age crater has focused on finding evidence for the interaction of water with rim materials, including implications for habitability (e.g., Arvidson et al., 2014, 2016; Squyres et al., 2012). The Cape York, Murray Ridge, and Cape Tribulation rim segments have been explored thus far (Figure 1). Traverses and measurements taken by Opportunity were in part directed based on findings from Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)-based hyperspectral observations, including detections of Fe3+ smectites on the eastern side of Cape York (Arvidson et al., 2014) and Fe3+-Mg2+ smectites in a gentle swale called Marathon Valley (Figure 2) on Cape Tribulation (Fox et al., 2016). Ground-based measurements with the Opportunity rover ’ ©2017. American Geophysical Union. during its kilometers of traversing on Endeavour s rim segments have included the acquisition of multispec- All Rights Reserved. tral (13F, 0.43 to 0.98 μm) images using the Pancam camera system on the rover’s mast (Bell III et al., 2006; STEIN ET AL. 278 Journal of Geophysical Research: Planets 10.1002/2017JE005339 Figure 1. Portion of a Mars Reconnaissance Orbiter (MRO) context imager (CTX, Malin et al., 2007) frame covering the southwestern part of Endeavour crater, includ- ing rim segments traversed (green line) and examined by the Opportunity rover. The blue box delineates location of a portion of an MRO HiRISE image covering Marathon Valley shown in Figure 2, where Opportunity conducted extensive measurements, including in a soil-filled fracture scuffed by the rover’s left front wheel. CTX frame f02_036753_1772_02s005w.tiff. Farrand et al., 2016, 2017), together with Microscopic Imager (MI, 31 μm pixel widths) (Herkenhoff et al., 2003) and Alpha Particle X-Ray Spectrometer (APXS) measurements (Gellert et al., 2006; Mittlefehldt et al., 2016) using the Instrument Deployment Device (IDD) to place these two instruments onto soil and rock targets. The Rock Abrasion Tool, also located on the IDD, has been used to brush and/or abrade selected rock targets before acquisition of MI and APXS measurements (Gorevan et al., 2003). Marathon Valley is 10–15 m deep, ~100 m wide, and strikes ~N60E across Cape Tribulation (Figure 2). It was the site of an extensive imaging and contact science measurement campaign (Crumpler et al., 2016; Figure 2. A portion of a HiRISE merged RGB image (McEwen et al., 2007) is shown covering Marathon Valley, located on Cape Tribulation. Opportunity entered Marathon Valley from the west near Spirit of Saint Louis Crater, acquired numerous remote sensing and contact science observations, and then entered Marathon Valley for another extensive set of measurements. One of the last measurement campaigns was on the bedrock target Pierre Pinaut and the nearby and scuffed soil-filled fracture. Image center location 2°18053.76″S, 5°21010.72″W. HiRISE frame ESP_036753_1775_MRGB_prj_v2.tiff. STEIN ET AL. 279 Journal of Geophysical Research: Planets 10.1002/2017JE005339 Figure 3. Portion of a Pancam false color mosaic acquired within Marathon Valley looking toward the southeast at the breccia outcrops and fractures filled by dark soils. The location of the contact science target Pierre Pinaut and the scuffed soil-filled fracture are shown before work was done on these targets. The black box delineates postscuff Pancam image view shown in Figure 4. The white arrows denote the margins of the scuff. Note specifically the red pebbles within the scuffed fracture. RGB shown using bands L2 (0.753 μm), L5 (0.535 μm), and L7 (0.432 μm). Pancam mosaic was acquired using a number of individual frames on sol 4343. Mittlefehldt et al., 2016). The entrance to Marathon Valley contains a shallow, flat-floored ~25 m diameter depression named Spirit of Saint Louis Crater (Figure 2). The depression is surrounded by an apron of bright planar outcrops cut into polygonal plates by fractures. These planar outcrops extend into Marathon Valley proper, and all of the planar outcrops are fine-grained impact breccias, likely a facies of the Shoemaker formation that dominates the outcrops on the western rim of Endeavour crater. The bulk compositions of the bedrock exposures found during Opportunity’s extensive measurement campaign at Spirit of Saint Louis Crater and Marathon Valley are similar to that of Shoemaker formation outcrops encountered elsewhere on Cape York and Cape tribulation, with slight enrichments in Mg and S (Mittlefehldt et al., 2016). Clusters of red pebbles with enhanced Al and Si contents, some exhibiting Pancam-based hematite spectral signatures (~0.86 μm absorption, Farrand et al., 2017), were found within some of the fracture zones located between the planar outcrops (Mittlefehldt et al., 2016). The planar outcrops are also interpreted to be the carriers of a CRISM-based Fe3+-Mg2+ smectite signature (Fox et al., 2016). 1.1. Scuff Zone Measurements After completing in situ measurements on a number of outcrop targets within Marathon Valley, Opportunity conducted extensive work on the Pierre Pinaut outcrop target, including abrading the target using the rock abrasion tool (RAT), followed by MI and APXS measurements. Opportunity was then commanded to drag its left front wheel across a nearby soil-filled fracture zone with a cluster of red pebbles at its edge (Figures 3 and 4). The scuff exposed a mixture of relatively bright, colorful soil with portions cemented into clods (Figure 4). Ten APXS observations were collected on or near the scuff: three overlapping measurements of a cluster of red pebbles (target George Drouillard) within the fracture and adjacent to the scuff zone, three overlapping measurements of colorful soil within the wheel scuff (target E Cann), and four overlapping measurements centered on a soil clod (target Private Joseph Field) (Figures 4 and 5 and Table 1).